Field of the Invention
The present invention relates to the field of flexible lithium-ion (Li-ion) batteries. In particular, the present invention relates to a method for preparing a self-supporting flexible electrode composition that requires neither the use of a synthetic polymer binder nor the use of organic solvents. The invention relates also to a negative or positive flexible composite electrode obtained by the implementation of said method, as well as a lithium-ion battery comprising at least one such flexible electrode.
Description of Related Art
Flexible Li-ion batteries can be used, like the conventional Li-ion batteries, in numerous devices which comprise portable appliances, such as, in particular, mobile phones, computers and light tools, or heavier appliances such as two-wheeled transport means (bicycles, motorized bicycles) or four-wheeled transport means (electric or hybrid motor vehicles). Generally, the flexible batteries can be used in all the applications in which it is desirable for the battery to be able to be deformed or to be folded in order, for example, to fill the empty spaces in hybrid or electric cars or to supply pliable electronic devices other than in all the conventional applications of rigid Li-ion batteries.
A conventional lithium-ion (Li-ion) battery comprises at least one negative electrode (anode) and at least one positive electrode (cathode) between which is placed a solid electrolyte or a separator impregnated with a liquid electrolyte. The liquid electrolyte consists, for example, of a lithium salt in solution in a solvent chosen to optimize the transport and dissociation of the ions. In particular, in a lithium-ion battery, each of the electrodes generally comprises a current collector (metal substrate) on which is deposited a composite material which comprises a material that is active with respect to the lithium, a polymer which acts as binder (for example a copolymer of vinylidene fluoride (PVdF)), an agent conferring electronic conductivity (such as, for example, carbon black) and a solvent.
During the operation of the battery, lithium ions pass from one of the electrodes to the other through the electrolyte. During the discharging of the battery, a quantity of lithium reacts with the active positive electrode material from the electrolyte, and an equivalent quantity is introduced into the electrolyte from the active material of the negative electrode, the lithium concentration thus remaining constant in the electrolyte. The insertion of the lithium into the positive electrode is compensated by the input of electrons from the negative electrode via an external circuit. During charging, the reverse phenomena take place.
The operation of the flexible Li-ion batteries is the same as that mentioned above for the conventional Li-ion batteries. However, in order to obtain a flexible or pliable battery, it is important to also develop electrodes that not only exhibit a strong conductivity, but also in which the layer of active material exhibits a strong adhesion to the substrate, which makes it possible to avoid the appearance of cracks, even the lifting of the active material after the flexing of the battery or, preferably, to develop self-supporting electrodes which do not require the use of a metal substrate.
Various methods for manufacturing flexible electrodes have been proposed in the literature. In particular, anodes exhibiting an enhances flexibility have been obtained by the filtration of suspensions of carbon nanotubes [S. Y. S. Chew, et al., Carbon, 2009, 47, 2976-2983], by pulsed laser ablation of silicon on carbon nanotube films [S. Chou, et al., The Journal of Physical Chemistry, 2010, C 114, 15862-15867] or even by laminating flakes of graphite in order to obtain graphite sheets used as anode [M. Yacizi, et al., Journal of Power Sources, 2005, 141, 171-176]. However, the methods developed above present drawbacks; they require the use of expensive raw materials, they take a long time to implement and consume a lot of energy or, in a few cases, the prepared electrodes exhibit weak electrochemical properties compared to the conventional electrodes.
The use on nano-Si/cellulose composite anodes comprising cellulose fibres as inert filler and demonstrating a good electrochemical stability has also already been proposed [J. L. G. Camer, et al., Electrochimica Acta, 2009, 54, 6713-6717]. However, the composite anodes of this publication were prepared using an organic solvent. Furthermore, the mechanical characteristics have not been determined and the application of these electrodes in a flexible Li-ion battery does not appear possible to envisage.
Furthermore, a synthetic polymer binder and an organic solvent are generally used to manufacture the electrodes. However, these compounds are not very environmentally friendly. Now, the current trend is, on the contrary, to find production techniques which have the weakest possible impact on the environmental front and to obtain devices/batteries which are easy to recycle.
To remedy these drawbacks, it has recently been demonstrated that microfibrillated cellulose dispersed in water can be used as binder for manufacturing graphite-based electrodes by boiling a suspension of graphite and of microfibrillated cellulose into a mould and eliminating the water by evaporation. This method thus avoids the use of synthetic polymers, of organic solvents and of metal substrates. However, the low speed of evaporation of the water and the high energy consumption required to produce microfibrillated cellulose represent a brake for the manufacture of these electrodes on an industrial scale [L. Jabbour, et al., J. Mater. Chem., 2010, 20, 7344-7347].
It has also been proposed to prepare a microfibrillated cellulose (produced by a Clodophora algae)/polypyrrole (PPy) composite material for the manufacturing of flexible batteries entirely based on organic materials. However, the low energy by weight (25 Wh/kg) and energy by volume (40 Wh/L) densities of the batteries thus obtained (NaCl-PPy) do not allow a direct comparison with the lithium-ion batteries, whether in terms of electrochemical performance or of the applications targeted.